L.J. AYUKO *(*) Head of Range Management Division, Ministry of Agriculture, P.O. Box 30028, Nairobi, Kenya (East Africa).
Range condition is the starting point for our decisions as regards site evaluation. Range trend helps us to maintain range condition at a set level. The recognition of range condition and trend is based on condition ratings as influenced by the state of soil and vegetation. The value of indicator species is clearly shown in the range trend - they are used in site evaluation; stratification and land use according to productivity; to indicate the quality of management plans in use; and to determine a downward or upward trend and a soil or vegetation trend in range trend and condition.
Site evaluation: Parameters and methods
1. Ecological Status: range condition, trend recognition, and value of indicator species.
Nearly every country in Africa has some rangeland. It is to the best advantage of the countries concerned, therefore, to utilize their rangelands to full capacity. This would help to reduce pressure on the arable lands and would also promote economy.
Before utilization of rangelands is possible, however, and to avoid any serious repercussions which might lead to range deterioration, it is essential to know the ecological status of the rangelands. Any form of range utilization should be geared to sustained productivity and improvement.
2. Before any form of utilization can be embarked on, the first thing to be determined is the condition of the range. This determination can only be achieved after a thorough site evaluation. This will include ascertaining the state of the vegetation and soil. The main factors that need to be looked into are:
a. analysis of the plant community (plant species present and their relative abundance),
b. vigour of the selected species,
c. amount and dispersion of soil cover,
d. extent of soil erosion.
The factors can be grouped into two categories to give a view of the range condition based on:
a. vegetation condition,
b. soil condition.
3. The knowledge of the vegetation condition, as a guide to evaluating the range condition and its suitability, can be arrived at through a series of stages as outlined below:
a) Study of the species composition of the plant community involved;
b) The grouping of the various plant species as desirables, intermediates, and least desirables (depending on their palatability and their relative position in the climax vegetation).
Based on the above, their production as a representation of vegetation cover and vigour is calculated.
4. The soil condition can also be used as a criterion to determine the range condition. The soil condition can be determined by taking into consideration the plant density, i.e., basal area plus plant litter dispersion, which will also determine, to a great extent, the soil condition; properly dispersed vegetation is more effective in soil protection than clumpy vegetation.
Currently soil erosion will act as an indicator of soil stability, and this can be related to the character of plant cover. The combined effect of slope and plant cover will determine the amount of soil erosion, or susceptibility to its erosion, to a large extent. As the percentage slope increases and the plant cover decreases, the rate of erosion increases. The reverse is also true.
5. Having thus established the condition of the range, it is important that its utilisation and management should be followed closely so as to determine whether current management is directed towards range improvement or deterioration. In order to determine the trend, certain indicators can be observed, and the observations evaluated to indicate whether the range is improving or deteriorating.
Soil and the vegetation can be used to determine range trend:
Soil as indicator
Various pointers can indicate downward trend. Some of these are: rill marks; active gullies; alluvial deposits; soil pedestals; and exposed plant crowns and roots.
These are indicators of range deterioration because before any type of soil erosion can occur, vegetation cover must have been reduced greatly and the soil exposed to various elements of weather.
Upward trend can also be noted by using soil as an indicator if the above are reversed: i.e. if the gullies are stabilized by perennial vegetation, rill and gullying activity ceases and the bare spaces are covered by perennial plants. This will indicate an improving trend.
Plants as indicators
The plant species present can also act as indicators for range trend. Like the soil, they can indicate both downward and upward trend of the range. However, in this case more caution is needed, since not all the plant species present may act as indicators of range trend, for example, Pennisetum clandestinum, which is highly favoured but very sensitive to misuse. If during management it is observed that the highly desirable plant species are constantly diminishing, then this should be seen as a sign of deterioration. On the other hand, the palatable species might not disappear altogether, but due to heavy grazing be continually depleted of foliage, thus reducing their vigor. This, and invasion by unpalatable plant species, are other indicators of range overuse. Finally, failure in the increase of both the palatable and possibly the unpalatable plant species is an indication of unfavourable microclimate, and this can only indicate a downward trend of the range.
The upward trend (improvement) of range can be observed if the above conditions are reversed. These would lead to vigorous palatable species increasing rapidly and continuously invading new areas.
On the overall, the trend of the range should be determined by the way the majority of the indicators point; the trend in range condition should be checked at various stages.
The two types of range trends that should be noted are:
a) Apparent range trend
This can be established at the time of deciding the range condition. However, for management purposes periodic reappraisals are essential, and correct measures must be taken to rectify any mistakes.
b) Long-term range trend
Long-term trend can be properly observed in areas that contain significant amounts of preferred range (areas where livestock will tend to feed if all conditions are ideal). These preferred areas are sensitive to livestock management and other types of impacts, to an extent that data collected within plots established in them, when evaluated, provide a guide to the trend. The optimum times for collecting such data can range from five- to ten-year intervals. Data for such long-term trends are very vital; i.e. if lost, they are almost irreplaceable and therefore need to be protected at all costs.
6. Throughout the preceding discussion, plant indicator species have been mentioned quite often. Indicator species can be of value in two major ways:
a) They can be used to determine the range trend, together with other points as stated above. However, in this case the indicator species must be sensitive both to livestock management and to any change in microclimate;
b) They can be used to determine the range condition, since there is usually a correlation between the types of plant species that will grow in certain types of soils under the same climatic conditions.